The present invention relates to a lithium ion secondary battery and a manufacturing method thereof. More particularly, the present invention relates to the structure of a wound electrode group that does not use an independent separator, and a manufacturing method thereof.
Electrochemical batteries, such as lithium ion secondary batteries, include a separator that is interposed between a positive electrode and a negative electrode. This separator has the function of electrically insulating the positive electrode plate from the negative electrode plate, as well as the function of retaining an electrolyte. A micro-porous thin film sheet, composed mainly of polyethylene, is currently used as the separator in lithium ion secondary batteries.
Further, lithium ion secondary batteries include a wound electrode group which is prepared, for example, by laminating and winding the positive electrode, the negative electrode and the separator. Such an electrode group, wound so as to have an oval cross section, is also used in rectangular batteries.
Various winding methods have been proposed. For example, Japanese Laid-Open Patent Publication No. Hei 9-35738 proposes using a winding core that is preferable, when the electrode plates are thick, for eliminating the impact of a gap created by the edges of the thick electrode plates at the initial winding position.
Also, for example, Japanese Laid-Open Patent Publication No. Hei 10-106530 proposes forming a porous film, which comprises a filler and a binder, on both sides of at least one of the positive and negative electrodes, instead of the conventional structure of providing a separator between the positive and negative electrodes, in order to improve high-rate characteristics and cycle characteristics (see claim 15 and FIG. 2). Such an electrode group is described below with reference to FIG. 4.
An electrode group of FIG. 4 is composed of a winding core 1, a negative electrode 2, and a positive electrode 4. The winding core 1 consists of an upper winding core 1a and a lower winding core 1b. The negative electrode 2 consists of a negative electrode core material 2a and negative electrode active material layers 2b formed on both sides of the negative electrode core material 2a. Further, a porous film 3, comprising a filler and a binder, is bonded to each of the negative electrode active material layers 2b. The positive electrode 4 consists of a positive electrode core material 4a and positive electrode active material layers 4b formed on both sides of the positive electrode core material 4a. 
The core material is exposed at the inner end of the negative electrode 2, and the exposed core material is sandwiched between the upper winding core 1a and the lower winding core 1b. Then, the negative electrode 2 and the positive electrode 4 are wound around the winding core, with the positive electrode 4 between the negative electrode 2, to form a wound electrode group.
In the electrode group of FIG. 4, an independent separator is not provided between the negative and positive electrodes, and the porous film is bonded to both sides of at least one of the positive and negative electrodes. Thus, no separator is necessary, and the structure of the electrode group can be simplified. Further, since the porous films are integrated into the electrode, short-circuits resulting from the displacement of a separator can be suppressed.
However, in such an electrode group, the porous film does not have a structure capable of retaining the strength in the direction parallel to the electrode surface on which the porous film is bonded, unlike an independent separator such as a micro-porous thin film sheet. Therefore, when the positive and negative electrodes are wound to form an electrode group, the porous films may become broken or separated around the inner most turn of the wound electrode group, due to the strain exerted on the wound electrode plates or the gap created by the edges of the electrode plates. In this case, the negative electrode and the positive electrode often come in contact with each other to cause a short-circuit.
In view of the above, an object of the present invention is to provide a lithium ion secondary battery having an electrode group that can reduce the breakage of porous films, and a manufacturing method thereof.